[cse491] homework4 question

[C. Titus Brown]

On Sat, Sep 20, 2008 at 10:34:17PM -0400, Alex Nolley wrote:
-> I'm a little uncertain about some code I have in my homework 4 webserver.
-> Since our program has to catch any exceptions that occur in the threads, I
-> first make a thread t that contains the info to spawn a client connection.
-> Next, I spawn the thread in a try, except clause where the except will catch
-> any exception, print out it's information to standard output and then pass
-> and continue with a new thread. The code is similar to this:
-> 
-> while 1:
-> 
-> # Setup the connection stuff
-> 
-> t = threading.Thread(target=handle_connection, args(client_sock,))
-> 
-> try:
->             t.start()
-> except:
->             print "An exception happened:", sys.exc_info()[0]

-> 
-> Will this code allow for true mulit-threading or does the try, except clause
-> prevent that from happening?

Hey Alex,

good question!  You could try experimenting, but the short answer is
that true multithreading *does* happen, because exceptions raised
inside threads don't propogate outside the thread.  So the 'try/except'
in the above code only deals with the 't.start()' statement but not
anything in the 'handle_connection' function.  Threads are entirely
independent of the main code execution task.

In brief, you need to put the try/except into handle_connection.

Note that 'traceback.print_exc()' will print the normal traceback
output.

cheers,
--titus

p.s. This also points out an exception to the try/finally rule: 

  try:
     t.start()
  finally:
     # do something

Here, 'finally' is run immediately after t.start() in the originating
thread, and not in the thread 't'.

--titus
-- 
C. Titus Brown, ctb at msu.edu